Drafting Selesnya sounds easy, right? Surely you draft creatures, big creatures, with teeth! And you put them on the battlefield, right? You line 'em up next to each other, and you turn 'em sideways repeatedly, until your opponent is nought but a greasy stain on the floor.
It certainly sounds simple, but that's not really how it works out. Firstly, you start seeing the word "populate" on a bunch of the cards you're drafting, so you start giggling to yourself as you imagine the shenanigans that will ensue as more and more creatures appear randomly on your side of the table with tiny comical popping noises, but by the time the draft's over, you realize you haven't actually grabbed any token making cards, and instead just a bunch of cards that populate.
Looking through the Selesnya cards that make token creatures in Return to Ravnica, it looks like you should be golden, but a surprising number of them are uncommon, or even rare. While some populate cards are very tempting, like Trostani's Judgment, you'll need to prioritize picking up some of the more innocuous looking token making cards, like Eyes in the Skies, and most importantly, Centaur's Herald and Knightly Valor. Coursers' Accord will do, in a pinch, but ideally, you want the Herald getting your population started as soon as possible.
The token-makers, like Centaur's Herald and Knightly Valor, are highly valued to the Selesnya strategy. You need to be able to make that all-important token if you want populate to do its thing!
From there, you obviously want to get a bunch of Centaur Healers and Towering Indriks to help hold off the Rakdos and Azorius hordes, Sunspire Griffin to evade defences, and Giant Growth, Swift Justice, and Common Bond to help settle any mid-field disputes. Keening Apparition is very useful as a "bear" that provides an excellent answer to Stab Wound, and Ethereal Armor is surprisingly effective at giving your Centaurs just enough power to break through Door Keepers and Lobber Crews, and if you've managed to pick up enough Knightly Valors, creating absurdly large and unbeatable monsters.
In short, if you skimp on your token makers, you're going to have a bad time. Once you get 'em, get a-populatin', and get in there!